42 MARCH 2014 • WORLD AQUACULTURE • WWW.WAS.ORG pond water results mainly from plankton, but of course, changes in plankton density are usually the major reason for changes in turbidity in well-managed ponds. Applications of 10 to 14 per year typically were used in those experiments. The applications usually were made at 3- to 4-week intervals. Frequent applications were needed because phosphorus is rapidly removed through adsorption by bottom soil and nitrogen is lost by denitrification and ammonia volatilization. Fertilizer was formerly packaged in 100-lb (45-kg) bags. Fertilizer vendors decided to package a 20-20-5 fertilizer in 40-lb (»18-kg) bags as a convenience to pond owners who might have difficulty handling a 100-lb bag. A 40-lb bag of 20-20-5 fertilizer – often called fish pond fertilizer – contained the same amounts of nutrients found in a 100-lb bag of 8-8-2. A fertilizer, such as 20-20-5, often is called a complete fertilizer because it contains the three primary plant nutrients – nitrogen, phosphorus and potassium. The 20-20-5 fertilizer became very popular and it still sometimes is used to fertilize sportfish ponds. Nitrogen-fixing bacteria and algae thrive in fertilized ponds and, over time, organic matter accumulates in bottom soils. The amount of ammonia nitrogen released by bacteria decomposing organic matter increases as the organic matter concentration of bottom soil increases. Exchangeable potassium that accumulates in bottom soils of potassium-fertilized ponds is available to the water column through cation exchange. Fertilizer phosphorus is bound tightly in bottom soil and largely unavailable. Thus, nutrient recycling increases the availability of nitrogen and potassium – but not of phosphorus – in ponds. In ponds fertilized for several years with a complete fertilizer, nitrogen and potassium inputs could be lessened or ceased (Swingle et al. 1963) and phosphateonly fertilization used at a P2O5 application rate of 8 lb/acre (9 kg/ha). A more thorough investigation conducted in ponds receiving phosphate-only fertilization tested two nitrogen sources, ammonium sulfate and calcium nitrate, each at four nitrogen rates (0, 3, 6, and 9 kg/kg) (Murad and Boyd 1987). This study confirmed that nitrogen fertilization did not result in greater fish production in ponds that had received a complete fertilizer for several years. The original method for applying fertilizer to ponds was to broadcast the granules over the water surface near pond edges. Care was taken to avoid spreading the granules over areas where they would settle below the thermocline in thermally-stratified ponds, making nutrients unavailable to phytoplankton in the upper, illuminated layer of water. This practice did not solve the problem of delivering fertilizer phosphate to phytoplankton. Only 5 to 10 percent of the phosphate in fertilizer granules dissolved while particles settled through the water column. Dissolution occurred mainly while the granules rested on the bottom soil (Boyd 1981). Rapid adsorption of fertilizer phosphate by sediment renders it largely unavailable to phytoplankton in the water column. Although total hardness often is a suitable indicator about the need for liming in many areas, total alkalinity is a more reliable indicator for general use (Boyd 1990). The response to liming was originally unpredictable (Swingle 1947, Thomaston and Zeller 1961) because there was no procedure for estimating the amount of liming material that should be applied. A lime requirement procedure used for agricultural soils in Alabama was modified for use in fish ponds (Boyd 1974). This was a major advancement in sportfish pond management because it allowed a recommendation on the amount of agricultural limestone needed to increase total alkalinity and total hardness concentrations to 20 or 30 mg/L. A recent study (Viriyatum and Boyd 2011) suggested that the target total alkalinity concentration of 20 mg/L is possibly too low, because sunfish production tended to increase with greater alkalinity concentration (40 to 60 mg/L). The lime requirement method for ponds (Boyd 1974) is based on determining the amount of liming material needed to increase base saturation of bottom soil to about 80 percent. A modification of the procedure (Pillai and Boyd 1985) determines the amount of liming material necessary to completely base saturate bottom soil. This method results in a larger lime requirement recommendation (and greater total alkalinity in pond waters) and it probably should be used instead of the original method. Agricultural limestone particles greater than 0.15 mm (100 mesh) and especially those larger than 0.25 mm (60 mesh) dissolve very slowly (Silapajarn et al. 2004). Samples of coarsely-ground and finely-ground agricultural limestone are depicted in Figure 1. When applied to ponds of similar initial alkalinity in equivalent amounts, the finer liming material increased total alkalinity to 45 mg/L while alkalinity increased only to 23 mg/L in the pond treated with the coarser material (Boyd 1990). Pond owners should purchase the most finely-ground material locally available. Agricultural limestone is not highly soluble and should be spread over the entire pond surface so that it will settle uniformly over the bottom. When pond management companies are contracted to apply limestone, they often load liming material onto a small barge and wash it into the pond with a high-pressure water hose (Fig. 2). Fertilization Rates. Initial studies showed that plankton productivity and fish production in ponds could be increased by fertilization (Swingle and Smith 1938, Smith and Swingle 1938). After conducting trials for several years with various fertilizers and nutrient application rates, Swingle (1947) and Swingle and Smith (1947) concluded that sportfish ponds should be fertilized at 100 lb/acre (112 kg/ha) per application of 8-8-2 (percent N, percent P2O5, percent K2O) grade fertilizer at intervals based on plankton abundance, as assessed by water clarity. A Secchi disk visibility of about 45 cm was considered adequate and the need for a fertilizer application was indicated by a greater Secchi disk visibility. This procedure is not reliable unless the turbidity in FIGURE 2. Samples of finely-ground (left) and coarsely-ground (right) agricultural limestone.
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